This invention relates to the recovery of useful components of fly ash derived from the burning of pulverized coal or from similar materials, by flotation.
Coal fired power plants generate fly ash from the combustion of pulverized coal, or combinations of pulverized coal and other carbonaceous supplemental fuels such as petroleum coke, woodbark, charcoal, wood, residual fibers, etc. The fly ash is captured in the power plant's emission control devices, such as by electrostatic precipitators and baghouses. The principal composition of fly ash includes an inert mineral fraction consisting primarily of ferro-alumino-silicate glass and residual unburned carbon from coal and/or other supplemental fuels.
The inert mineral fraction of fly ash is a pozzolan, which makes fly ash an acceptable mineral admixture for use in Portland cement concrete. The fly ash itself includes the devolatized mineral matter which had been trapped or loosely associated with the coal as well as incombustible components and elements of the coal and/or of the supplemental fuels. In addition to such incombustible components, the fly ash contains carbon rich particles which have not been completely combusted, usually due to the inefficiency of the boiler design or related conditions.
Specifications for the use of fly ash in Portland cement concrete are set out in ASTM #C-618. This specification limits the loss-on-ignition (LOI) content to fly ash pozzolan to less than 6%. The LOI value of fly ash is generally equal to the percent by weight of the unburned carbon content of the fly ash. This carbon content can vary from as little as about 0.5% up to 20% or more of the weight of the total fly ash product. However, for pozzolanic activity, a good quality fly ash should contain less than 1% carbon but, in any case, no more than about 4% carbon.
A high carbon fly ash as a pozzolanic mixture has a detrimental impact upon the quality of concrete. The presence of carbon reduces air entrainment, which, in many locations, is the only real protection which concrete has against freeze-thaw/wet-dry conditions. Therefore, the lower the carbon content, the better the concrete mix is from an air entrainment perspective. The presence of carbon also increases water requirements, reduces pozzolanic reactivity, and degrades the appearance of finished concrete surface. Thus, carbon negatively affects the strength, durability, and aesthetic appearance of concrete. Therefore, the lower the carbon content in any fly ash the better the fly ash as a concrete admixture.
The presence of high levels of unburned carbon and fly ash has been exacerbated by the use of nitrous oxide emission control apparatus at the fly-ash generating plants, and also by the economic incentive of power plants to use less expensive supplemental fuels, such as Columbian coals, and/or by the use of petroleum coke or to burn paper or tissue mill waste such as wood, woodbark, wood chips, and residual fibers.
Recent investigations have shown that the carbon particles themselves, are porous and tend to entrap small fly ash particles called microspheres. These microspheres, which are usually less than 1 micron in size, tend to occupy the surface pores of the devolatized carbon particles. While such spheres are highly desirable as a component of a fly ash mixture, when entrapped within the carbon content they contribute little or nothing to the cementitous reaction. Therefore, if the carbon fraction can be effectively and economically separated and removed from the fly ash and, if in doing so, the microspheres can be released and utilized as part of the pozzolanic material, then a more pure carbon fraction and an enhanced pozzolan may be realized.
Flotation has been suggested as a viable process for removing or reducing the carbon fraction in fly ash. In conventional carbon flotation systems, high carbon fly ash containing 4% or more unburned carbon is prescreened and mixed with water to make a 20% to 65% solids by weight slurry and conditioned in a mixing tank for 10 to 20 minutes using a collector reagent. The reagent normally consist of a hydrocarbon such as petroleum distillates premixed with surfactants such as petroleum sulfonates. The surfactants emulsify the petroleum distillates and promote their dispersion throughout the ash slurry. The emulsified and dispersed petroleum distillates have the affinity to adsorb onto carbon particles rendering them hydrophobic. The hydrophobic carbon particles are then floated and transported to the slurry surface with air bubbles and are trapped in a dark froth layer generated with the use of a frother reagent such as 2-ethylhexanol and other alcohols and glycols. The two slurry streams recovered from the flotation process; (1) carbon rich float and (2) pozzolan tailing are separately dewatered and dried prior to marketing.
Such processes are disclosed in Brewer U.S. Pat. No. 3,794,250 issued Feb. 26, 1974, Hurst et al. U.S. Pat. No. 4,121,945 issued Oct. 24, 1978, Hwang U.S. Pat. No. 5,074,145 issued Sep. 10, 1991 and Groppo et al. U.S. Pat. No. 5,456,363 issued Oct. 10, 1995. All such processes depend upon mechanical conditioning prior to flotation. Typically conditioning up to one-half hour or more, as described in Hurst et al. Also, prolonged mechanical conditioning in water is believed to reduce or degrade the pozzolanic quality of the fly ash pozzolan fraction.
Thus, the conditioning step is costly both in terms of time and energy input. Also, such processes, to provide good separation in the flotation cells, use a surfactant or emulsifier in addition to the petroleum based reagent. The surfactant, such as a petroleum sulfonate, for example emulsifies the petroleum distillate and promotes dispersion. The use of an emulsifier adds to the cost of the process.